Batteries, in particular high-performance batteries based on nickel metal hydride or lithium, such as those frequently used in today's applications for driving vehicles, for example electric vehicles or hybrid vehicles, are typically made of stacked prismatic modules. Multiple modules of this type are then combined to form the battery. The individual modules include the electrochemical cells of the battery, multiple cells of this type typically being provided in a single module.
Furthermore, the modules typically have an electric connection at each of their narrow, opposite ends. The stack of individual modules forming the battery is then usually joined under tension via mechanical end plates and tension members to form the battery. In addition to mechanically attaching the modules to each other, the end plates and tension members are used, in particular, to counteract deformation due to gas pressure changes which occur in the electrochemical cells provided in the interior of the modules during operation. High-performance batteries of this type also require cooling to ensure the necessary operating temperature within the battery.
For example, a battery of this type having prismatic modules is known from EP 1 117 138 A1, in which spacers, for example corrugated sheets or the like, are inserted between the individual modules. This produces channels between the individual modules through which a coolant, in particular air, can flow for the purpose of cooling.
In the case of this convection cooling, the outer areas of the individual electrochemical cells located in the modules are cooled to a greater extent than the inner central areas, due to the larger contact area with the coolant. As a result, the electrodes placed on the outer surfaces of the module are cooled to a greater extent than those closer to the inside of the module. In addition, the electric discharge line, which is situated in the area of the outer cells and typically also conducts heat, acts in such a way that the outer module cells are cooled to a greater extent. Finally, the individual modules of a battery of this type are cooled at different rates, since the modules positioned in the area of the end plates also undergo greater cooling than the modules in the middle of the stack, due to the cooler end plates in their direct vicinity.
The uneven cooling of the individual electrochemical cells in the module then results in uneven cell voltages during module operation and in a substantially faster aging of the warmer cells in the center of the module. This aging is expressed in an increase in the internal resistance and a related decrease in the capacitance of the individual cell. However, since the individual electrochemical cells are typically connected in series, the usable overall capacitance of the battery is no greater than the capacitance of the electrochemical cell having the lowest capacitance. The use of the battery, for example in a vehicle, can therefore be limited to a great extent by an aging-related loss in the capacitance of a single cell. The best way to counteract this effect requires highly complex and, in particular, large cooling devices, which are difficult to accommodate in motor vehicles, especially in the small amount of space available.
Furthermore, the use of air as a cooling medium, for example in the aforementioned EP publication, has the disadvantage that high-volume air channels, air filters, and fans are needed to conduct the quantity of air needed for cooling. It is further problematic to implement an air cooling system, since the input temperature of the air used should not be excessively high for effective cooling. For use in a vehicle, this means, in particular, that the battery cooling is dependent on the conditions of the cooling air. For example, battery cooling is not as efficient at extremely high ambient temperatures as it is at much lower ambient temperatures.
An alternative to the cooling method explained above is described in U.S. Pat. No. 5,756,227. In this case, conduction cooling is practiced instead of convection cooling for primary heat removal. For this purpose, cooling fins are integrated between the individual modules. These cooling fins terminate in a common base element, which, in turn, is cooled by convection. This convection can be implemented, for example, by air or by cooling lines having a liquid cooling medium.
A problem with the design according to the aforementioned US publication is that uniform mechanical joining of the stack of modules under tension is no longer possible, due to the rigid cooling fins. When gas pressure builds up in the individual cells, the outer housing walls of the modules therefore begin to bulge. As a result, air gaps, which result in loss of thermal contact between the module and cooling fins, form between the module and cooling fin, due to the module deformation caused by the pressure rising therein. This means that uniform cooling can also not be ensured. Once again, this results in the aforementioned disadvantages of poor and, in particular, uneven cooling of the individual module cells.
To avoid the aforementioned problem, EP 1 278 263 A2 describes a structure of a prismatic battery in which metallic cooling fins are integrated into the material of the modules.
The aforementioned structure at least attempts to avoid the aforementioned problems resulting from the loss of contact between the cells and cooling fins. However, it has the disadvantage that achieving sufficient stability in a material structure of this type having integrated metal plates requires very thick materials. However, these very thick materials, which are typically made of plastic material forming the casing of the metal plates, have the disadvantage that they are poor heat conductors.
Furthermore, in the case of the integrated metal plates according to the aforementioned EP publication, the thermal coupling of the metal plates to the cooled base plate is extremely difficult. The position of the heat-removing contacts of the integrated metal plate on the base plate changes during battery operation, depending on the gas pressure in the individual modules as well as the elasticity of the tension members connecting the individual modules to the stack via the end plates. The heat-removing contacts of the integrated metal plates must therefore be able to slide on the cooled base plate. This function alone, which is necessary as a result of the unavoidable, pressure-related expansion in the battery stack, makes it clear that maintaining clean and uninterruptible conduction of heat from the integrated metal plates to the base plate for all phases of operation will be very difficult.
A further disadvantage is certainly the fact that constructing the modules from different materials having varying coefficients of thermal expansion is critical. Modules of this type also frequently undergo highly dynamic temperature fluctuations in the range of 20 K to 100 K. Due to the different expansion coefficients of the integrated metal plates, compared to the module material, which is typically made of plastic, the plastic easily peels off the integrated metal plates. Even this construction therefore results in air gaps which substantially impair thermal conductivity and also significantly reduce the mechanical stability of such modules, which is important due to the pressure fluctuations.